Fluorescent inks and toners are among the most widely used security printing features. A printed document is usually authenticated by detecting the light emitted by the fluorescent component when subjected to black light. The light emitting property cannot be reproduced in a second generation copy.
Fluorescent dyes used in fluorescent inks and toners can lose fluorescence in the print-head when the ink is heated to a temperature greater than 120° C. to melt during normal operation. To overcome this problem, the security printing industry uses hard, robust pigments containing the dye of interest. Pigments are preferred over fluorescent dyes because of their improved chemical, light fastening and thermal stability. Pigments are also preferred by the industry because there is limited or no migration or bleeding of the dye compound.
Most commercially available fluorescent pigments are made by grinding a bulk polymer matrix containing fluorescent materials. This process does not result in fluorescent particles of a size smaller than 1-2 microns, and typically the size of these particles is about 4-5 microns. According to this process, fluorescent dyes are incorporated into hard, crosslinked particles, thereby limiting the mobility of the fluorescent dye. Once the fluorescent dye is isolated from interaction with other materials present in the ink and, chemical degradation by the environment is diminished. These hard particles are dispersed in the marking material, typically liquid inks.
Inks based on fluorescent pigments are currently used in rotogravure, flexographic, silk-screening and off-set printing systems. However, given their large size, pigments cannot be used with inkjet, solid ink or UV curable inks, because they physically clog the inkjet nozzles. In addition, pigments are unsuitable for fabrication of Emulsion Aggregation (EA) toners since the size of the fluorescent particles is about the size of the toner particles.
There is a need in the art for fluorescent compositions, including fluorescent compositions that can be used in/with inkjet, solid ink, UV curable inks and EA toners and that have suitable thermal degradation properties. There is a further need for fluorescent compositions of such small size that can be used in/with inkjet, solid ink, UV curable inks and EA toners and are compatible with organic based marking materials.